This invention relates to patterned optical interference filters, a preferred method for producing them and the use of such filters with lamps. More particularly, this invention relates to optical interference filters of a predetermined pattern or geometry, continuous or discontinuous, symmetric or asymmetric and their use with lamps.
Multilayer optical interference filters and their use with electric lamps are well known to those skilled in the art. A commercially available, high efficiency lamp including an optical interference filter that has achieved considerable commercial success is the Halogen-IR.TM. lamp available from General Electric Company. Briefly, this lamp includes a miniature, double-ended, linear light source such as a halogen-incandescent light source, mounted inside a parabolic reflector. The light source is fabricated from a fused quartz envelope and has a multilayer optical interference filter disposed over the entire external surface of the envelope. The filter is transparent to visible light radiation but reflects infrared radiation emitted by the light source back to the light source. Each time the infrared radiation is reflected back to the light source, at least a portion is converted to visible light radiation which is then emitted by the lamp.
The optical interference filter is made of alternating layers of refractory metal oxides having high and low indexes of refraction. Refractory metal oxides are used in these types of applications because they are able to withstand the relatively high temperatures ranging from between about 400.degree.-900.degree. C. on the outer surface of the high temperature glass or fused quartz envelope that encloses a filament or arc source during operation. Such oxides include, for example, titania, hafnia, tantala, and niobia for the high index of refraction material and silica or magnesium fluoride for the low index of refraction material.
Multilayer optical interference filters are useful for hot mirrors and as cold mirrors on reflectors, and also as coatings or films on reflectors, lamps and lamp lenses to alter the emitted or projected color as desired. It is desirable to be able to apply such optical interference filters to the surface of the filament or arc chamber envelope of a lamp or onto the surface of an outer lamp envelope, reflector or lens in a predetermined asymmetric or symmetric pattern to selectively reflect and transmit various portions of the electromagnetic spectrum in a predetermined direction and pattern.
Relatively large, conventional incandescent lamps having a metallic coating symmetrically disposed on the glass envelope for reflecting the emitted light in a desired direction or pattern are known in the prior art. The reflector materials disclosed in known arrangements, though, are deemed deficient for a number of reasons. For example, known reflector arrangements are not capable of withstanding high temperatures in excess of 400.degree. C. or are only applied in geometrically symmetric and continuous configurations. Many applications require a light source (e.g. halogen or arc tube) that has a power density above four watts per square centimeter (4 watts/cm.sup.2). If a reflective coating was disposed on an external surface of the light source, then known coatings would be inadequate since the coatings would not withstand the high temperatures associated with such a power density range. Also many known coatings will reflect the heat, but with optical interference coatings selectivity with regard to transmitted light, e.g. wavelength, color, heat emission, or U.V. control of the light are exemplary of a few variables that can be controlled.
Prior arrangements sought to maximize the light emitted in a beam by spatially enveloping as much of the light source as possible with a reflector. In order to concentrate the beam in small angle compact structures, and simultaneously provide low magnification of the projected image, reflectors had to be quite large. In recent years, though, there has been a growing demand for more compact directional lighting systems for use in various applications such as automotive and display lighting.
One way to address the concern with reflector size is to use a low profile, truncated parabolic reflector. Headlamps are one common commercial product where truncated parabolic reflectors are used in that manner. Unfortunately, a portion of the light emitted by the source does not reach the active portion of the reflector, i.e., the parabolic surface portion. With a linear light source aligned with a central axis of the parabolic reflector between upper and lower truncating reflecting surfaces, light emanating upwardly or downwardly from the light source and directly reaching the upper and lower truncating surfaces is wasted. In contrast, light emanating rearwardly so as to reach the parabolic reflecting surface is controllably directed to achieve a desired beam pattern. Light emanating directly forward from the light source, and bypassing all reflecting surfaces, lacks the directional control provided by the parabolic reflecting surface and results in glare to an observer. Truncation results in collection inefficiency and decreased beam candlepower. To counteract this, it is often necessary to increase the source power.
The Halogen-IR.TM. lamp developed by General Electric Company and mentioned above overcomes some of the drawbacks of the reduced collection efficiency of compact, truncated reflectors. The provision of an infrared (IR) light reflective coating applied on and covering the entire outer surface of the envelope increases efficacy of the filament tube source.
While the IR reflective coating is more desirable than prior arrangements, it still suffers the same loss in collection efficiency and beam candlepower as the reflector lamp is made more compact. The truncated automotive headlamp arrangement described above is but one example. Other, and a wide variety of, light systems can be improved.
Accordingly, a need exists for a high intensity type of incandescent, arc discharge, or electrodeless lamp having a multilayer optical interference filter disposed on the outer surface of the light source envelope in a predetermined pattern for selectively reflecting and transmitting desired portions of the electromagnetic spectrum emitted by the light source in a predetermined direction and pattern. It would be desirable to provide a partially coated light source having a compact means for causing a greater extent of the light generated by the source to be projected in predetermined orientations and patterns, for example, onto a reflecting surface of a lighting system.
The present invention contemplates a new and improved process for coating a lamp, a coated lamp and lighting systems employing the coated lamp that overcome all of the above referenced problems and others while simultaneously satisfying various objectives in an economical manner.